1. Field of the Invention
This invention relates to lenses for use in optical disc players.
2. Description of the Prior Art
Optical discs are known which consist of a rigid transparent disc with a reflective coating on the surface remote from the read head of the player. Non-reflective dots are created in the reflective coating to create a record of information. The record is read by directing a laser beam at the reflective coating, through the transparent disc, and sensing the reflected energy, whereby the dots may be detected by virtue of their low reflectivity.
The laser beam is focussed on the plane of the reflective surface by a lens and reflected energy is focussed on a sensor by the same lens. The laser beam must be very accurately focussed on the reflective surface, within the transparent disc, and the player must maintain this exact focus as the disc rotates at high speed in front of the lens. The disc may not be quite planar, there may be some slop in the rotational mount of the disc, and ambient conditions may change. All these factors lead to a necessity for adjusting the focus which is effected by moving the lens. Usually, the focus adjustment has to be very rapid. Rapid movement of the lens is easier the less the mass of the lens.
It is an object of the present invention to provide a lens for an optical disc player, which lens has good resolution and low mass.